It was a “watershed” attack, “one in a million,” an all-but-unforeseeable “black swan.”

In the aftermath of the mass shooting at the Las Vegas country music festival, event security professionals — many with years of experience thwarting bad actors in bustling crowds — are characterizing the ambush in darkly exceptional, almost fatalistic terms. But they are also reckoning with ever-changing threats in their field after the aerial assaults that killed at least 59 people and injured more than 520 on Sunday.

The specter of calamity is especially worrisome for open-air events in urban environments — including the Austin City Limits music festival, which begins Friday in a Texas park and is now undergoing renewed security assessments.

“There is no manual for this,” said Chris Robinette, the president of Prevent Advisors, a security subsidiary of Oak View Partners, a company that advises sports and entertainment venues like Madison Square Garden. “It is a dynamic process that requires promoters, venue managers, local authorities and other stakeholders to work together.”

Ever since the Sept. 11 attacks, standard security protocol at concerts, festivals and other large entertainment events has become increasingly sophisticated, mirroring the mainstream adoption of previously unheard-of safety precautions at airports around the country. Music gatherings — long bastions of ephemeral intimacy and relaxed inhibitions — have become the site of bomb-sniffing dogs, body scanners and high-definition closed circuit cameras, particularly in the wake of recent large-scale attacks on concerts including the Bataclan rock club in Paris and the Manchester Arena.

Jeff Dorenfeld, a music business professor at Berklee College of Music with a focus on touring and festivals, recalled a time when concert security barely existed. “I was at Altamont,” said Mr. Dorenfeld, who went on to tour with Ozzy Osbourne and Boston, of the 1969 concert infamous for deaths and violence. “There wasn’t real security.”

But even with the gradual ratcheting up of protections, a new wave of mass casualty events has highlighted the ways determined attackers can wreak havoc by shifting their focus to the areas immediately surrounding venues.

In Las Vegas, the gunman, Stephen Paddock, executed his killing spree from a towering window at the Mandalay Bay Resort and Casino — around 400 yards away from the festival site — well outside the usual security perimeter of pat-downs and metal detectors that is created for such events. He slipped the hotel’s own security apparatus, and chose an open-air target that is by definition vulnerable from a high elevation.

“We have to go back to Lee Harvey Oswald on the book depository to conjure a similar scenario,” said Steven A. Adelman, vice president of the Event Safety Alliance, by telephone Monday afternoon, referring to the assassination of President John F. Kennedy. “This really is unique.”

Louis Marciani, the director of the National Center for Spectator Sports Safety and Security, struggled to imagine how a similar assault might be prevented in the future. “There’s no way that any good operation would have caught that,” he said of the shooting. “We’ve now got to go back to the drawing board.”

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The Austin City Limits Music Festival in 2011. The 2017 festival begins on Friday. After the Las Vegas shooting, the Austin police chief said, “Whenever you have an incident occur you always have to be concerned about copycats.”CreditBen Sklar for The New York Times

Las Vegas Village, the site of Route 91 Harvest Festival, is owned by the same company — MGM Resorts International — as the hotel where Mr. Paddock opened fire. It is likely that there was at least some preplanning between the two facilities before the festival took place. (MGM has not commented.) But even if Mandalay Bay were on high alert last weekend, snaring Mr. Paddock would most likely have required a level of screening that far exceeds current practices.

“You’d have to have X-ray machines and magnetometers at every single entrance,” said Mr. Adelman. “No hotel does that.”

Festival organizers could choose to avoid locations near the sorts of tall buildings that can offer gunmen cover and a clear vantage, but Mr. Adelman suggested that other loopholes would then emerge. “Do you not hold festivals near hills or tall trees?” he wondered. “Do you ban trucks?”

Mr. Dorenfeld offered a similarly rueful hypothetical. Does every festival now have to be like Bonnaroo, “in the middle of an open field?” But he stressed that procedures are constantly evolving. “I go to these festivals and I look around and I’m so impressed,” he said. “Security is really good and it’ll just get better.”

For the people behind Austin City Limits, which will bring 75,000 music fans to Zilker Park in Austin, Tex., this weekend, the question of how to keep people safe is now freighted with even more pressing urgency than usual.

Security professionals and some event promoters have called for expanding the perimeter around so-called soft targets, and for increased coordination between venues and neighboring businesses.

Following the Las Vegas shooting, the producer of the Austin festival, C3 Presents, released a statement detailing a “layered security plan that includes elements that are seen and unseen,” and that will include “an enhanced security and law enforcement presence inside and outside the festival.”

The company also announced that it would offer refunds to any fans concerned about safety.

The Austin police chief, Brian Manley, said on Monday: “It’s not that it’s a threat that we are not aware of, but whenever you have an incident occur you always have to be concerned about copycats — someone that looks at this as an opportunity.” In a news conference, he added that officers had already visited condos on the park’s north side that partly overlook the festival grounds.

Chief Manley struggled to buoy his tone while speaking to would-be festival attendees, though he urged them to continue to “do the things that we enjoy.”

But he ended the news conference with an unvarnished caveat. “We live in a world now where you cannot protect against every single threat,” he said.

A version of this article appears in print on , on Page A12 of the New York edition with the headline: ‘There Is No Manual for This’: Security Experts Reckon With Aerial Assault. Order Reprints | Today’s Paper | Subscribe